Traditional polyurethane-reaction injection molding (PU-RIM) has been, for many years, applied to selfskin foams, microcellular elastomers, shoe soles, furniture, decorative articles, automotive bumpers, and dashboards. Innovational usage on automotive exterior parts, such as hoods, the trunk doors, body covers, fenders and doors have been attempted but have not been very successful, mainly because the stiffness and heat-sag properties of traditional PU-RIM are not sufficient to endure on-line painting.
The chemicals used in traditional polyurethane reaction injection molding are generally in the liquid state at room temperature. They can be easily mixed at ambient temperature but the products have low rigidity and poor heat resistance and therefore are not considered as a suitable material for exterior automotive components.
It is well known that the polyurethane molecule is composed of two segments, namely, a hard segment and a soft segment, which influence its physical and mechanical properties. The structure and relative amounts of the hard and soft segments are mainly dependent on the ratio of the raw materials and structure of polyol and chain extender. The chain extenders used in traditional room temperature PU-RIM have a low melting point. These are generally in the liquid state at room temperature and include ethylene glycol, diethylene glycol, propylene glycol, dipropylene glycol and 1,4-butane diol. When they react with diisocyanate to build the hard segment in polyurethane molecular structure, poor physical properties result because of the nature of aliphatic molecular structure.